Pull in StreamIndex from vg to support indexed GAM

`vg` has acquired a concept of GAM indexing, for random access to node-ID-sorted GAM by node ID.

There's a bit of demand to read these files from tools that don't want to depend on vg in its entirety, so we should pull https://github.com/vgteam/vg/blob/master/src/stream_index.hpp and its dependencies over into this library from vg, so they can be more easily depended on in other tools.